THS3095_14 Datasheet, PDF (20/37 Pages) Texas Instruments – HIGH-VOLTAGE, LOW-DISTORTION, CURRENT-FEEDBACK OPERATIONAL AMPLIFIERS

THS3091
THS3095
SLOS423G – SEPTEMBER 2003 – REVISED OCTOBER 2008........................................................................................................................................ www.ti.com
F_IN
R_G
249 Ω
V_S
R_F
1 kΩ
27 pF
V_S
FB
_
+
−V_S
49.9 Ω
5.11 Ω
100-Ω LOAD
1 µF
Figure 67.
Figure 68 is shown using two amplifiers in parallel to
double the output drive current to larger capacitive
loads. This technique is used when more output
current is needed to charge and discharge the load
faster like when driving large FET transistors.
1 kΩ
249 Ω
24.9 Ω
V_S
_
+
−V_S
1 kΩ
5.11 Ω
V_S
V_S
1 nF
249 Ω
_
5.11 Ω
+
24.9 Ω
−V_S
Figure 68.
Figure 69 shows a push-pull FET driver circuit typical
of ultrasound applications with isolation resistors to
isolate the gate capacitance from the amplifier.
V_S
V_S
+
5.11 Ω
_
−V_S
866 Ω
191 Ω
866 Ω
V_S
_
5.11 Ω
+
−V_S
−V_S
Figure 69. PowerFET Drive Circuit
SAVING POWER WITH POWER-DOWN
FUNCTIONALITY AND SETTING
THRESHOLD LEVELS WITH THE
REFERENCE PIN
The THS3095 features a power-down pin (PD) which
lowers the quiescent current from 9.5 mA down to
500 µA, ideal for reducing system power.
The power-down pin of the amplifier defaults to the
positive supply voltage in the absence of an applied
voltage, putting the amplifier in the power-on mode of
operation. To turn off the amplifier in an effort to
conserve power, the power-down pin can be driven
towards the negative rail. The threshold voltages for
power-on and power-down are relative to the supply
rails and are given in the specification tables. Above
the Enable Threshold Voltage, the device is on.
Below the Disable Threshold Voltage, the device is
off. Behavior in between these threshold voltages is
not specified.
Note that this power-down functionality is just that;
the amplifier consumes less power in power-down
mode. The power-down mode is not intended to
provide a high-impedance output. In other words, the
power-down functionality is not intended to allow use
as a 3-state bus driver. When in power-down mode,
the impedance looking back into the output of the
amplifier is dominated by the feedback and
gain-setting resistors, but the output impedance of the
device itself varies depending on the voltage applied
to the outputs.
Figure 70 shows the total system output impedance
which includes the amplifier output impedance in
parallel with the feedback plus gain resistors, which
cumulate to 2380 Ω. Figure 59 shows this circuit
configuration for reference.
20
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